I'm having trouble finding out any errors in a ray diagram.

The question is as follows:

A ray of light falls normally on the curved face of a semi-circular plastic block, it hits the horizontal face of the block at an angle of incidence of 30 degrees, and emerges to air forming an angle of 40 degrees with the normal.

This drawing carries 6 marks, but the teacher circled a part of the following drawing and it fetched 5.5 marks.

However, I can't spot any errors. Are there are any issues in this diagram? Please let me know in either case, if you think there are, or aren't.

enter image description here

  • 1
    $\begingroup$ The problem state that a ray of light falls normally (ie at a 90 degree angle) on the curved face... however, your incident ray is definitely not normal to (perpendicular to) the curved face of the block where it hits. That would be my best guess. $\endgroup$
    – Sean
    Feb 23, 2015 at 19:40

1 Answer 1


A ray which is incident normal to an interface will have zero refraction (in first order optics). The angles in Snell's law are measure with respect to the normal of the interface and since $$n_1\sin\theta_1=n_2\sin\theta_2$$ if $\theta_1=0$, $\theta_2$ must also be zero.

That means that for the ray to strike the flat interface of the semi circle at an incident angle of $30^O$, the ray must enter at a location $30^o$ below the apex (or $60^o$ above the horizontal.)

(Your teacher was generous. ;)


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